1. Field of the Invention
The present invention relates to a hydraulic device used for a construction machine or the like. The hydraulic device has a plurality of directional valves which have a flow control function capable of controlling the pressure oil from a single hydraulic pump which flows into each of a plurality of actuators, and a plurality of pressure compensation valves for compensating the pressures of the respective directional valves.
2. Description of the Related Art
This type of hydraulic device is employed primarily for construction machinery and agricultural machinery; it is equipped with a load-sensing required-stream regulation function for controlling the delivery of a variable displacement pump according to loaded pressure. Further, the circuits connected to actuators are provided with pressure compensation valves to divide the pump delivery so as to prevent the respective actuators from interfering with each other due to the difference in loaded pressures, etc. among the respective actuators with a resultant change in speed of the actuators when driving the plurality of actuators at the sane time. Furthermore the hydraulic devices are equipped with a function known as an anti-saturation function for distributing pump delivery to the individual actuators at an appropriate ratio when the pump delivery is smaller than a predetermined required flow of the plurality of driven actuators, as disclosed, for instance, in U.S. Pat. No. 4,617,854 in which as a load-sensing required-stream regulation function, there is provided with a pump flow control valve which is adapted to cause a spring force and a maximum loaded pressure among the loaded pressures of the actuators to act in a direction for increasing the delivery of the variable displacement pump and to cause a delivery pressure to act in a direction for decreasing the delivery of the variable displacement pump in opposition to the foregoing acting forces, thus controlling the pump delivery according to loaded pressure. There has also been disclosed a hydraulic device in, for example, Japanese Patent Laid-Open No. 4-19409, wherein a pressure compensation valve is disposed on the downstream of a directional valve which has a flow control function, the pressure compensation valve is adapted in its respective control pressure chambers to cause a pressure (Pd') on the upstream side of the pressure compensation valve to act in a direction for opening the pressure compensation valve and is adapted to cause maximum loaded pressure (Pm) of actuators to act in a direction for closing the pressure compensation valve, thus providing the anti-saturation function.
According to the FIG. 2 of U.S. Pat. No. 4,739,617, a differential pressure control valve is provided to produce a secondary pressure (Pc=Pd-Pm) corresponding to the differential pressure between the delivery pressure (Pd) of a variable displacement pump and the maximum loaded pressure (Pa) of the actuators. The secondary pressure (Pc) supplied by the differential pressure control valve and an actuator loaded pressure (PL) which is a pressure on the downstream side of a directional valve both are adapted in its another respective control pressure chambers to act in a direction for opening the pressure compensation valve, and a pressure (Pz) on the downstream side of a pressure compensation valve is adapted in its another respective control pressure chambers to act in a direction for closing the pressure compensation valve. Further, Japanese Patent Laid-Open No. 4-54303 has disclosed in its FIG. 6 a hydraulic device which is equipped with a differential pressure detector for detecting the differential pressure between the delivery pressure (Pd) of a variable displacement pump and the maximum loaded pressure (Pm) among actuators, a controller for generating a control signal in response to an output received from the differential pressure detector, and an electromagnetic proportional valve which is actuated by the control signal generated and outputted by the controller and which outputs the secondary pressure (Pc) so as to secure the anti-saturation function.
The flow characteristic in of the pressure compensation valve which has such a conventional anti-saturation function is indicated by a balance between the operating pressure to open the pressure compensation valve in its control chambers and the operating pressure to close the same. A differential pressure between the loaded pressure (PL) of an actuator and a pressure (Pz) on the upstream side of a directional valve, that is, a differential pressure .DELTA.P before and after the directional valve (hereinafter referred to as directional valve differential pressure),i.e. the directional valve differential pressure being expressed as: .DELTA.P=Pz-PL=Pd-Pm=Pc
The directional valve differential pressure is adapted to be proportional to the differential pressure between the delivery pressure of the variable displacement pump and the maximum loaded pressure of the actuator, i.e. the secondary pressure.
It is known, however, moving a load with high inertia in a hydraulic device which carries out the pressure compensation described above causes an unstable operation of a system with consequent hunting. For instance, the hunting is noticeable in operating a hydraulic excavator of a construction machine, when moving a swing motor for a cab or travel motors for crawlers with heavy load or a boom cylinder or other cylinder with heavy load, posing a problem of impaired operability. Specifically, an example is taken wherein a lever of a directional valve is moved by a certain amount in steps to operate an actuator, namely, a swing motor for a cab or the like, with high inertia. Firstly, the directional valve is opened to let oil to flow into an actuator; however, the actuator does not immediately move because the actuator has high inertia, thus causing the loaded pressure to rise momentarily. The rise in the loaded pressure causes the loaded pressure to act on the pressure compensation valve to widely open the pressure compensation valve. Thus, the actuator which has received a large flow is suddenly accelerated; however, the acceleration gradually attenuates although the speed increases, because the supply of the flow is limited once the actuator is started. Hence, the loaded pressure, which has suddenly risen, gradually goes down as the acceleration decreases; therefore, the opening of the pressure compensation valve accordingly grows smaller gradually and the flow supplied decreases. When the actuator loses the acceleration and reaches a constant speed, the constant speed is significantly higher than a target speed since the speed has resulted from the high acceleration at the start, whereas the then loaded pressure is considerably low since the acceleration has already attenuated. This causes the opening of the pressure compensation valve to become even smaller with a consequent lower differential pressure of the directional valve. Hence, the flow decreases and the actuator starts to slow down, but the actuator attempts to keep the speed because of the high inertia thereof, causing the loaded pressure to decrease further. This in turn causes the opening of the pressure compensation valve to be even smaller with a resultant even slower speed of the actuator; however, when the speed has decreased to a certain level, the loaded pressure gradually recovers and the opening of the pressure compensation valve gradually grows larger accordingly. The deceleration of the actuator eventually stops and reaches a constant speed; however, the constant speed is considerably lower than the target speed since it has gone through the sudden decrease at the early stage of the deceleration. At the same time, the then loaded pressure is back to a large level since the deceleration has stopped; therefore, the opening of the pressure compensation valve is large again and the differential pressure of the directional valve is accordingly back to high, thus causing the actuator to start accelerating. Once the actuator begins to accelerate, the same initial phenomenon mentioned above takes place again. Thus, the repetitious sudden acceleration and the sudden deceleration hardly fade and the hunting continues. In actual operation, the response delay of a pump device is added, resulting in a further complicated phenomenon. Thus, the circuit using pressure compensation valves has been posing a problem in that the hydraulic control system using the circuit tends to develop unstable operation and hunting when moving a load with high inertia. In the device disclosed in U.S. Pat. No. 4,617,854 or U.S. Pat. No. 4,739,617, the operating pressures in the opening and closing directions in each of respective control chambers of the pressure compensation valve are set to be equal; and the devices are also equipped with the anti-saturation function; however, no prevention of hunting has been disclosed or suggested. In a device disclosed in Japanese Patent Laid-Open No. 4-54303, an extremely small pressure receiving area in a control pressure chamber of the pressure compensation value is used to cause the delivery pressure of a main pump to act in a direction for opening the pressure compensation valve, so that when the differential pressure between the delivery pressure and the loaded pressure of an actuator increases, the outlet flow of the pressure compensation valve is increased to cancel a flow force so as to secure an outlet flow which is not affected by the flow force, thus preventing the hunting or unstable operation of the pressure compensation valve which is caused by the reduction in the outlet flow due to a flow force generated by the throttle part of the pressure compensation valve when a plurality of actuators are operated at the same time. Again, however, no preventive measures for a load with high inertia have been disclosed or suggested.
Furthermore, in all the prior art described above, the maximum loaded pressure (Pm) have been acting in a direction for closing a pump flow control valve which varies the displacement of the pump via a thin or a small diameter, long pilot line from a valve unit. Hence, when the viscosity of pump delivery oil increases at low temperature with a resultant excessive pressure loss in the line from the pump to the value unit, the pressure on the upstream side of the pressure compensation valve in the valve unit drops by the foregoing pressure loss. This causes the differential pressure of the directional valve to drop, significantly reducing the pump delivery flow supplied to the actuator.
When at least two actuators, out of the plurality of actuators must be driven in synchronization with each other regardless of the loaded pressure of the actuators as in a case where two travel motors for driving a pair of crawlers of a hydraulic traveling vehicle are run, control is performed by the pressure compensation valve so that the directional valve differential pressures before and after the directional valves will be equal by shifting the levers of the respective directional valves by the same stroke; therefore, it is expected that if equal flow is supplied to the respective travel motors, the hydraulic traveling vehicle will be able to travel straight. If, however, there is a machining error in the spools of the directional valves will, the openings of the throttles of the individual directional valves inevitably be different even when the differential pressures of the directional valves are made equal. This means that the flow supplied to the respective travel motors will not be the same. Likewise, if there is an error in the pressure receiving areas resulting from the machining errors in the pressure compensation valves, the differential pressures of the directional valves will not be equal even when the respective openings of the throttle of the individual directional valves being shifted by the same. This stroke are the same, posing a problem in that the hydraulic traveling vehicle is unable to travel straight.
Furthermore, when at least two hydraulic actuators with markedly different loads are operated at the same time, such as a swing hydraulic motor and a hydraulic boom cylinder of a hydraulic excavator for a cab, the excessive inertial load of the actuator with a higher load causes an excessive pressure to be generated at an actuator port at the inlet in the early stage of the simultaneous operation. As a result, most of pressure oil flows from an overload relief valve, which is installed at the actuator port at the inlet, into a tank, causing an effective delivery flow itself to be reduced. This has presented a problem in that the driving speed of the boom cylinder which is the hydraulic actuator with a lower load becomes extremely slow and a large energy loss of an engine results from the pressure oil flowing into the tank from the relief valve. After that, when the acceleration of the swing motor stops and a constant speed is reached, the loaded pressure of the swing motor suddenly drops. The pressure compensation valve for the swing Rotor is almost fully open by the excessive loaded pressure of the swing motor in the early stage; however, this opening suddenly becomes small as the loaded pressure suddenly drops. This has presented a problem in that the swing motor is unavoidably accompanied by a shock when it decelerates, and because this deceleration enables an additional (effective) delivery of the pump to be acquired, the boom conversely accelerates, resulting in an awkward motion.